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Natural pest control: Tree species diversity increases spider density

Despite their reputation as creepy critters, spiders actively serve as some of the most environmentally friendly pest regulators. Spiders actively feed on flies, moths, roaches, and mosquitoes and thus playing a critical role in eliminating parasites and various disease-vectors. 

According to a recent study led by the University of Maryland (UMD), a simple way to take advantage of these natural ecosystem services is to provide tree-dwelling spiders a more diverse habitat. 

“We found that there’s a strong link between the species diversity of tree habitats and the population density of the spiders that live in them,” said senior author Karin Burghardt, an assistant professor of Entomology at UMD. 

“Spiders really like complex habitats, so having a large variety of tree species with different structural features like height, canopy cover, and foliage density will help increase spider abundance and also the natural pest regulation they provide.”

Studying the role of tree species 

This recent study is part of a larger project called BiodiversiTREE, led by the Smithsonian Environmental Research Center, and consisting of a series of large-scale, long-term experiments aiming to restore coastal forests along compromised shorelines of Chesapeake Bay by investigating the role of tree species diversity in forest responses to climate change, from the molecular to the watershed level.

“There have been some large-scale tree diversity experiments conducted in Europe and Asia, but our study is one of only a handful of experiments of its kind in temperate North America. With this research, we’re discovering crucial information about our local ecosystems, especially regarding how they react to both climate change and restoration efforts,” Burhardt explained.

How scientists studied tree species and spiders 

The researchers reforested 75 plots of formerly agricultural land with 16 tree species native to Maryland, including red maple, white oak, and black gum trees. Each of these plots had an area of 35 square meters and contained 255 trees. 

To examine how changes in tree diversity affect ecosystems, the scientists planted either the same tree species, four, or 12 different tree species on the plots.

From 2019 to 2021, the scientists repeatedly sampled 540 trees in these plots by counting the number of spiders from each tree.

What the researchers learned

Ultimately, the team discovered that plots with higher tree species diversity hosted greater spider populations. For instance, researchers found that plots containing four or 12 different tree species supported 23 to 50 percent more spiders than plots with a single species.

“Plots with more variation in tree species tend to have more canopy cover than plots with only a single tree species,” Burghardt said. “More canopy cover and tree shade may mean more water retention, cooler temperatures, additional hiding spaces from predators, and better web-making environments for spiders – all features that influence spider distribution.”

Researchers found the strongest relationship between tree diversity and spider populations during the late summer months, when temperatures were at their highest.

“Spider development time and reproduction is strongly influenced by changes in temperature. In addition, the summer months are also when their prey, insect herbivores, are the most active. This means maintaining certain microclimate conditions is essential to helping to maintain spider population levels and their natural pest control services, which allow managers to minimize the use of toxic chemical pesticides,” Burghardt explained.

Implications of the tree species study

As global temperatures continue to rise to due climate change, the number of tree-feeding insect pests and occurrences of potentially deadly diseases transmitted by vectors such as mosquitoes or ticks are likely to also increase. 

Thus, designing habitats that support spiders and other beneficial organisms could offer environmentally and economically sustainable ways to counteract this worrisome trend.

“Tree planting initiatives can be used as strategic responses to changes in our local ecosystems, but they can also be proactive maneuvers. Research like this will help us plan more resilient forests for the future,” Burghardt concluded.

More about spiders

Spiders are a large, diverse group of predatory arthropods belonging to the order Araneae within the class Arachnida. Scientists document over 48,000 different species of spiders, and regularly discover new species.

General description

Spiders exhibit significant variations in size and appearance. The smallest known species, the Patu digua, measures less than 0.37 millimeters in length, while the largest, the Goliath birdeater tarantula, can reach sizes up to 28 centimeters in leg span.

Despite these variations, spiders share several key characteristics: they all possess eight legs, two main body sections (the cephalothorax and the abdomen), and fangs that inject venom. They also typically have eight eyes, although some species have fewer, and some cave-dwelling and parasitic spiders have none at all.

Web building spiders

Spiders are famously known for their ability to spin intricate webs from silk, a material produced in their abdominal silk glands. Different spiders create different types of webs, including the classic orb webs, funnel webs, cobwebs, and sheet webs.

However, not all spiders weave webs. For instance, wolf spiders and jumping spiders prefer to actively hunt their prey rather than trapping it in a web.

Feeding and predation

The primary diet of spiders consists of insects and other small arthropods. Spiders employ various hunting strategies, such as trapping prey in their webs, ambushing prey, or actively hunting.

Most spiders paralyze or kill their prey using venom delivered through their fangs. Afterward, they consume their meal by injecting digestive enzymes into the prey’s body to liquefy its tissues, then sucking up the resulting fluid.

Reproduction and life cycle

The process of spider reproduction typically begins with a complex courtship ritual, which often involves the male demonstrating that he is a mate rather than prey.

After successful mating, the female produces one or more egg sacs, which can contain hundreds to thousands of eggs.

Spiderlings emerge as miniature versions of adults, and they gradually increase in size through a process known as molting.

Distribution and habitat

Spiders inhabit every continent except Antarctica and thrive in nearly every ecosystem, from the hottest deserts to the coldest mountain tops. They also adapt well to human environments, including homes and gardens.

Venom and spider bites

While all spiders produce venom, only a few species are dangerous to humans. Notable species include the brown recluse and the black widow in North America, and the Sydney funnel-web spider in Australia.

Despite this, the vast majority of spider bites are harmless to humans, typically resulting in no more than a minor, temporary reaction.

Ecological importance

Spiders play a crucial role in the environment as predators of insects and other small arthropods, helping to control populations of these organisms. Their predatory activities benefit humans by reducing populations of pests that can damage crops and spread disease.

In addition, the silk produced by spiders has a range of potential applications, from medical sutures to body armor materials, owing to its remarkable strength and elasticity.

In summary, spiders, while often misunderstood and feared, are an integral part of our planet’s ecosystems. Their diversity, predatory efficiency, and silk production abilities make them a fascinating group of creatures worthy of study and respect.

Through their ecological role and potential applications in medicine and materials science, spiders underscore the interconnectedness of life and the importance of biodiversity.

The study is published in the journal Ecology.

By Andrei Ionescu, Staff Writer

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